Vacuum Tube Op Amp Experiments
June 20, 2015 6:59 pm ProjectsAt the electronics flea market I recently found something particularly interesting…
This is a vintage Philbrick K2-W vacuum tube operational amplifier! Turns out they have quite the following. Sadly the original 12AX7 tubes were gone–somewhere, someone probably has rare GAP/R marked tubes in their guitar amplifier. I put in some generic replacements.
I decided to build a little jig to try it out.
This is based on the inverting amplifier schematic given in the K2-W datasheet I linked above. I added a simple linear power supply to generate the +/-300V rails. If you build your own supply, be sure to add bleeder resistors so you don’t get a nasty surprise after you turn it off and try to work on it.
After connecting a 10K series and 100K feedback resistor to the op-amp, I ran a 1KHz 5Vp-p square wave from my function generator into the circuit and saw this:
Neat! The top trace is the input and the bottom trace is the output. The bottom trace has a magnitude of 50Vp-p, as expected.
It’s really interesting to see how the short paragraph of specifications at the bottom of the first page of the K2-W datasheet developed into the formal electrical characteristics tables you can see in more modern op-amp datasheets, like the 741.
Hedley :
Date: June 21, 2015 @ 11:20 am
Amazing bit of history. Thanks for showing this!
Olivier Bauwens :
Date: February 16, 2017 @ 3:56 pm
Gave you schametric of the power suppley. Thanks
eric :
Date: February 25, 2017 @ 1:12 am
It’s just a Hammond transformer connected to a bridge rectifier made out of 1N4007s and some “big” (>470uF) electrolytic capacitors.